The paper presents a new diagrammatic approach to analyzing first-order iterative algorithms, which include power iteration, belief propagation, and gradient descent methods. The key insights are:
The state of the algorithm at any iteration can be expressed as a linear combination of diagrams, which are small unlabeled graphs. The operations of the algorithm correspond to simple combinatorial operations on these diagrams.
In the limit as the input size n goes to infinity, only the tree-shaped diagrams contribute to the asymptotic behavior of the algorithm. The tree diagrams represent asymptotically independent Gaussian random variables.
Using the diagram basis, the authors are able to rigorously justify several heuristic arguments from statistical physics, such as the equivalence between belief propagation and approximate message passing (AMP) algorithms, as well as the state evolution formula for AMP.
The authors also show that the tree approximation holds for a surprisingly large number of iterations, up to nΩ(1) for the case of debiased power iteration. This goes beyond the natural boundary suggested by a naive analysis.
The diagrammatic approach provides a unified and flexible framework for analyzing a wide class of first-order iterative algorithms, bridging the gap between the rigorous mathematical analysis and the powerful heuristic techniques developed in statistical physics.
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